The aim of this study was to determine plasma concentration of magnesium, potassium, calcium and sodium in children with mitral valve prolapse (MVP) and to establish the relation between electrolyte abnormalities and ventricular arrhythmias (VA). The study group consists of the 113 children with MVP (age +/- SD: 10.7 +/- 4.3 years). The 101 healthy children (age +/- SD: 13.0 +/- 3.2 years) were enrolled into the control group. All patients underwent clinical examination, standard ECG, 24 hr ECG Holter monitoring and echocardiography. Blood samples were taken 3 times in fasting state in separate days. All patients were in sinus rhythm. Cardiac arrhythmias were observed more frequently in children with MVP (53%) than in control group (31%, p < 0.0008). VA were recorded in 39% MVP patients, compared with 13% of the healthy children (p < 0.0001). There were no differences in K, Na and Ca concentrations in children with MVP comparing to control group. Serum Mg concentration was significantly lower in MVP group (0.88 +/- 0.08 mmol/l) comparing to healthy children (0.91 +/- 0.07 mmol/l, p < 0.002). A marked decrease in Mg concentration was recorded in patients with MVP and ventricular arrhythmias compared with those without ventricular arrhythmias (0.83 +/- 0.07 vs 0.90 +/- 0.06 mmol/l, p < 0.0001). This study demonstrates that potentially serious dysrhythmias are common in pediatric patients with MVP. It would appear that ambulatory ECG monitoring should be part of the complete evaluation of children with MVP. Since decreased plasma concentration, magnesium supplementation seems to be a prudent approach in children MVP with frequent ventricular premature beats and in children with ventricular tachycardia.